The present patent application claims the right of priority under 35 U.S.C. xc2xa7119 (a)-(d) of German Patent Application No. 10124209.3, filed May 18, 2001.
The present invention relates to biaxially stretched multilayer polyolefin films having a base layer, at least one intermediate layer, and at least one peelable outer layer, in which the intermediate layer is interposed between the base layer and the peelable outer layer. The present invention also relates to a method of preparing such multilayer films, in which the base and intermediate layer(s) are biaxially stretched, and the peelable outer layer(s) is monoaxially stretched.
In stretched polyolefin films, in particular biaxially oriented polypropylene films, the mechanical strength of seal seams is often higher than that of the film itself, and therefore when a sealed pack is opened it is not only the seal seam which is broken apart. Typically, a tear propagates in an uncontrolled manner through the entire film upon opening. Peelable films are films which once sealed can be separated again mechanically without damaging or destroying the film itself. Peelable seal layers are typically required to have good sealing properties and also permit controlled opening of the pack. An important feature of this type of peelable seal layer is that this behaviour arises over a very large range of temperature of the sealing tools, and that the seal seam strength is very constant over this temperature range. The seal seam strength for stretched polyolefin films here must not be excessively high, since these films have very low tear propagation resistance and tear apart in an uncontrolled manner when subjected even to the most minor damage. Another important feature is the requirement of good film transparency, this being desirable for many applications in order to give the product complete or partial visibility and in some instances to provide good legibility of markings. An example of where transparency is desirable is with the packaging of compact discs.
Another important feature is good hot seal seam strength. This is necessary since, especially when producing bags on vertical tubular bag machines, directly after sealing of the basal seam the product to be packaged drops onto the basal seam while it is still hot. The resultant mechanical load placed upon the bag""s basal seam, which is still hot, must not lead to any damage to the seam, nor indeed cause the bag to break open.
U.S. Pat. No. 4,666,778 discloses transparent, peelable films with high seal seam strength. The peelable layers have a thickness of 40 xcexcm or above. Seal layers of this thickness are disadvantageous for oriented polyolefin films, since they make them unsuitable for many applications where thin films are required.
EP-A-0 692 373 and EP-A-0 781 652 describe opaque films with at least one peelable outer layer. However, a transparent film is desirable for many applications.
U.S. Pat. No. 5,500,265 discloses a peelable film. The coating is composed of an acrylic polymer or of a polyvinylidene chloride polymer. A disadvantage of this film is that the cut material cannot be recycled. The film is expensive to produce, since coating has to take place in a separate operation after film production, and often requires an additional adhesion promoter.
The object was therefore to provide a stretched multilayer polyolefin film with high transparency, which is peelable after sealing at various temperatures and thus has a wide peel range. The film also has a high hot seal seam strength and very constant seal seam strengths over this peel range.
In accordance with the present invention, there is provided a peelable, biaxially oriented, transparent multilayer polyolefin film, having at least three layers, comprising:
a) a transparent polyolefin base layer;
b) at least one peelable outer layer comprising one of,
(i) a polymer selected from at least one of LLDP, LDPE, MDPE, ethylene-vinyl acetate copolymer, ethylene-methyl acrylate copolymer and HDPE, and
(ii) a combination of at least one propylene co- or terpolymer and at least one ethylenic polymer; and
c) at least one intermediate layer comprising a propylene co- or terpolymer,
wherein said intermediate layer c) is interposed between said base layer a) and said peelable layer b).
Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, etc. used in the specification and claims are to be under stood as modified in all instance by the term xe2x80x9cabout.xe2x80x9d
The film of the invention has at least three layers and encompasses, as substantive layers, always: the transparent base layer, which is preferably composed of polypropylene; at least one intermediate layer made from propylene co- or terpolymer; and at least one peelable outer layer, preferably outer layers on both sides, which may be identical or different. In the case of a single-side peelable outer layer the opposite outer layer is preferably a sealable outer layer. Among the numerous materials used for sealable layers, preference is given to the use of the polymers selected from at least one of: random propylene-ethylene copolymers; random propylene-1-olefin copolymers; and random propylene-ethylene-olefin terpolymers.
A particularly preferred propylene-ethylene copolymer for use in the optional sealable layer is characterized in that it contains from 1.0 to 10% by weight of ethylene. The density of the seal layer polymer is preferably in the range of 0.895 to 0.960 g/cm3, and, depending on the type, the crystalline melting point is preferably from 120 to 150xc2x0 C.
In an embodiment of the present invention, the multilayer film has four layers, one of which is a sealable outer layer d). More particularly, in the four-layered film: the transparent polyolefin base layer a) is interposed between the sealable outer layer d) and the intermediate layer c); and the intermediate layer c) is interposed between the transparent polyolefin base layer a) and the peelable outer layer b). The structure of this four layer film is represented by the following:
|sealable layer (d)|base layer (a)|intermediate layer (c)|peelable layer (b)|
If the film of the invention has two peelable outer layers, the film also preferably has two intermediate layers, location of each of these being between the outer layers and the interior base layer. If the film of the invention has only one peelable outer layer, the film also typically has only one intermediate layer between the peelable outer layer and the base layer.
In an embodiment of the present invention, the multilayer film comprises five layers, in which two outer peelable layers (b), two intermediate layers (c) and one transparent base layer (a) are present. More particularly, in the five layered film: the transparent polyolefin base layer a) is interposed between two intermediate layers c); and each of the intermediate layers c) is interposed between a peelable outer layer b) and the transparent polyolefin base layer a). Each of the two separate intermediate layers c) may have the same or different compositions, and each of the two separate peelable outer layers b) may have the same or different compositions. The structure of this five layer is represented by the following, in which xe2x80x9cint.xe2x80x9d means intermediate, and xe2x80x9cpeelxe2x80x9d means peelable:
|peel layer (b)|int. layer (c)|base layer (a)|int. layer (c)|peel layer (b)|
The overall thickness of the multilayer film of the present invention may vary within wide limits. In a preferred embodiment, the multilayer film has an overall thickness of from 10 to 100 xcexcm, and more preferably from 15 to 40 xcexcm. The thickness of the peelable outer layer(s) is preferably in the range from 0.5 to 8 xcexcm. The thickness of the intermediate layer(s) made from propylene co- or terpolymer is preferably in the range from 0.5 to 5 xcexcm.
The base layer is preferably substantively or completely composed of a polypropylene. It is preferable to use an isotactic polypropylene with density of from 0.90 to 0.91 g/cm3 and with a melt flow index of from 1 to 4 g/10 min at 230xc2x0 C. and 21.6 N (to DIN 53 735). The film may also comprise lubricants and antistats.
Lubricants that may be used in the multilayered film of the present invention include, for example, higher aliphatic amides, higher aliphatic esters, waxes and metal soaps, and also polydimethylsiloxanes. The effective amount of lubricant is in the range from 0.01 to 3% by weight, preferably from 0.02 to 1% by weight. Addition of amounts of higher aliphatic amides in the range from 0.01 to 0.25% is particularly suitable. Erucamide is a particularly suitable aliphatic amide.
Preferred antistats that may be used in the multilayered film of the present invention include, for example, alkali metal alkanesulphonates, polyether-modified, i.e. ethoxylated and/or propoxylated polydiorganosiloxanes (polydialkylsiloxanes, polyalkylphenylsiloxanes and the like) and/or the substantively straight-chain and saturated aliphatic, tertiary amines having an aliphatic radical having from 10 to 20 carbon atoms, where these have substitution by o)-hydroxy-(C1-C4)-alkyl groups, particularly suitable compounds being N,N-bis-(2-hydroxyethyl)-alkylamines having from 10 to 20 carbon atoms, preferably from 12 to 18 carbon atoms, in the alkyl radical. The effective amount of antistat is typically in the range from 0.05 to 0.5% by weight. Glycerol monostearate is also a suitable antistat, the amount used being from 0.03 to 0.5% by weight.
In a preferred embodiment, the peelable layer is substantively composed of LLDPE, LDPE, MDPE, ethylene-vinyl acetate copolymer, ethylene-methyl acrylate copolymer, HDPE or a mixture of these. The ethylenic polymers mentioned preferably have an ethylene content of from 80 to 100% by weight, with preference from 95 to 100% by weight. LDPE and LLDPE are particularly suitable.
In another preferred embodiment, the peelable layer is substantively composed of a mixture made from at least one propylene co- or terpolymer and from at least one ethylenic polymer, e.g. LLDPE, LDPE, MDPE, ethylene-vinyl acetate copolymer, ethylene-methyl acrylate copolymer or HDPE. The ethylenic polymers mentioned preferably have an ethylene content of from 80 to 100% by weight, with preference from 95 to 100% by weight. LDPE and LLDPE are particularly suitable.
The propylene co- or terpolymers of the peelable layer generally have a melt flow index of from 1.5 to 30 g/l 0 min, preferably from 3 to 15 g/10 min (to DIN 53 735). The melting point is in the range from 120 to 140xc2x0 C.
In a preferred embodiment of the present invention, the intermediate layer c) comprises, and is preferably composed substantially of, at least one of: (i) a copolymer selected from at least one of a copolymer of propylene and ethylene, a copolymer of propylene and butylene, and a copolymer of propylene and another olefin having from 5 to 10 carbon atoms; and (ii) a terpolymer selected from at least one of a terpolymer of propylene, ethylene and butylene, and a terpolymer of propylene, ethylene and another olefin having from 5 to 10 carbon atoms.
The propylene co- or terpolymers of intermediate layer c) generally have a melt flow index of from 1.5 to 30 g/l 0 min, preferably from 3 to 15 g/10 min (DIN 53 735). The melting point is typically in the range from 120 to 140xc2x0 C.
The films of the invention may be produced by conventional processes, such as lamination, coating, or melt coextrusion. Once the high-gauge film has been extruded and solidified on the casting roll, the film is stretched in the machine direction (longitudinally) with an orientation factor of from 4/1 to 7/1 at a temperature of from 120 to 150xc2x0 C. The transverse stretching ratio is preferably from 8/1 to 12/1, and the transverse stretching of the film is carried out at a temperature of from 130 to 170xc2x0 C. To ensure that the substantially non-polar film surface has affinity for printing inks, it is useful to subject the film to corona (discharge) pretreatment in a manner known per se, thus incorporating atmospheric oxygen in the form of carbonyl, epoxy, ether or alcohol groups on the film surface. Other methods of pretreatment for polypropylene films are pretreatment using flame, plasma or fluorination methods.
In accordance with the present invention, there is further provided a process for producing the multilayer film, which comprises:
(A) coextruding, through a flat-film die, the base layer a) and the intermediate layer c) to form a coextruded first film comprising the base layer a) abutting at least one intermediate layer c);
(B) drawing the first film off of at least one roller and solidifying the first film;
(C) stretching the solidified first film in the direction in which it is drawn off of said roller (or in the direction of running of the machine);
(D) applying the peelable layer b) to the intermediate layer c) of the stretched first film by means of a combination of exposure to heat and pressure, thereby forming a second film; and
(E) stretching the second film in a direction perpendicular to the stretching of step (C), thereby forming the biaxially oriented multilayer film of the present invention.
When in an embodiment of the invention the multilayer film includes an exterior sealable layer d) the exterior layer is coextruded with the base layer a) in step (A) of the process.
The result of the above process is that the base layer and the intermediate layer(s) have been stretched biaxially, but the peelable outer layer(s) have been stretched only monoaxially, i.e., perpendicularly to the direction of running of the machine.
The film of the invention has good peel properties, good hot seal seam strength, and high transparency. The seal seam strength remains at a substantially constant level over a wide temperature range, giving the packaging machine wide processing latitude.
Test Methods:
Determination of Seal Seam Strength:
This was determined by placing two strips of width 15 mm one on top of the other, and using a sealing time of 0.5 s and a sealing pressure of 50 N/cm2 for sealing at the respective temperature. The force needed to separate the seal seam was then measured.
Determination of Hot Seal Seam Strength:
Hot seal seam strength was determined on a vertical tubular bag machine from the company Bosch (model SVZ 1650 AM), by packing 500 g of dried peas at 50 cycles/min with a sealing temperature of 140xc2x0 C. (profile across the seal seam). The drop height for the peas is 1 m. The test is passed if the seal seams are not damaged by the impact of the peas.